Astronomers rely on clear observations to study celestial objects, but cosmic dust can alter our observations, making stars appear redder and dimmer than they actually are. To correct this, researchers need to understand how dust interacts with light.
Astronomers used data from ESA's Gaia mission and high-resolution spectra from the LAMOST survey to create the most detailed three-dimensional map of dust in the Milky Way. This breakthrough not only enhances our ability to observe distant stars, but also deepens our understanding of interstellar dust, which plays a crucial role in the formation of stars and planets. Unexpectedly, their study revealed that the death of dust behaves differently than previously thought, which could hint at the presence of complex molecules such as polycyclic aromatic hydrocarbons, which could have implications for the origin of life.
Celestial obstacles caused by cosmic dust
When we observe distant stars, what we see is not always what we get. A star that looks redder may not actually be that color, and its light may have been altered before it passes through cosmic dust and reaches our telescopes. To ensure the accuracy of observations, astronomers must take into account this dust, which not only makes the star's color reddish (a phenomenon known as "reddening") but also dims its brightness ("extinction"). It's like we're looking at the universe through a dusty window. Now, two astronomers have created a groundbreaking three-dimensional map that reveals the distribution and properties of interstellar dust in unprecedented detail, providing new clarity to our view of the universe.
How dust is changing our view of the universe
Fortunately, astronomers have ways to reconstruct the dust's effects. Cosmic dust absorbs and scatters light unevenly, affecting shorter wavelengths (blue light) more than longer wavelengths (red light). This pattern, known as an "extinction curve," helps scientists determine the dust's composition and understand its environment, including radiation conditions in different regions of interstellar space.
Retrieve dust information from 130 million spectra
Zhang Xiangyu, a doctoral student at the Max Planck Institute for Astronomy (MPIA), and Gregory Green, leader of the independent research group at the institute (Sofia Kovalevskaya's group) and Zhang Xiangyu's doctoral supervisor, used this information to construct the most detailed three-dimensional map of dust properties in the Milky Way to date. Zhang and Green used data from ESA's Gaia mission, which lasted 10.5 years and made extremely precise measurements of the positions, motions and other properties of more than 1 billion stars in the Milky Way and our nearest galactic neighbor, the Magellanic Clouds. The Gaia mission's third data release (DR3), released in June 2022, provided 220 million spectra, and quality checks told Zhang and Green that about 130 million of them were suitable for their search for dust.
Gaia spectra have low resolution, that is, the way they separate light into different wavelength regions is relatively crude. The two astronomers found a way around this limitation: 1% of the stars they selected have high-resolution spectra provided by the LAMOST survey conducted by the National Astronomical Observatory of China. This provides reliable information about the star's fundamental properties, such as its surface temperature, which determines what astronomers call the star's "spectral type."
Reconstructing a 3D map of interstellar dust
Zhang and Green trained a neural network to generate model spectra based on the properties of stars and the properties of interstellar dust. They compared the results to 130 million suitable spectra from Gaia and used statistical ("Bayesian") techniques to infer the properties of the dust between us and these 130 million stars.
These results allowed astronomers to reconstruct the first detailed three-dimensional map of the extinction curve of galactic dust. The map was made possible because Zhang and Green measured the extinction curves of an unprecedented number of stars - 130 million stars, while previous studies only measured about 1 million stars.
Dust is more than just a nuisance to astronomers, though. It is important for star formation, because stars form in giant clouds of gas that are obscured by dust from surrounding radiation. When stars form, they are surrounded by disks of gas and dust, which are where planets are born. Dust grains themselves are the building blocks that eventually become planetary solids like our Earth. In fact, in the Milky Way's interstellar medium, most elements heavier than hydrogen and helium are locked away in interstellar dust grains.
Unexpected properties of cosmic dust
The new results not only produce accurate three-dimensional maps. They also discovered a surprising property of interstellar dust clouds. Prior to this, it was expected that the extinction curve in regions of higher dust density should become flatter (less dependent on wavelength). Of course, in this case, the "higher density" is still very small: about one billionth of a gram of dust per cubic meter, or just 10 kilograms of dust in a sphere with the radius of the Earth. In such areas, dust particles tend to grow in size, changing the overall absorption characteristics.
Instead, the astronomers found that in the medium-density region, the extinction curve actually becomes steeper, with absorption at small wavelengths being more efficient than absorption at longer wavelengths. Zhang and Green speculate that the cause of this steepening may not be dust, but the growth of a class of molecules called polycyclic aromatic hydrocarbons (PAHs), which are the most abundant hydrocarbons in the interstellar medium and may even have played a role in the origin of life. They have set out to test their hypothesis with future observations.
Compiled from /ScitechDaily